Virtual reality systems are designed to present a user with computer-generated visual, and optionally, auditory cues which allow the user to experience a computer-simulated reality. At the same time, the user may be equipped with a motion-responsive glove or body suit which permits user interaction with the virtual reality information being presented. For example, the user may be able to touch and "move" an object in the visual field, or experience a changing auditory cue as the image of an object in the visual field is moved with respect to the user's ear.
One important application of virtual reality systems is in flight simulators, where visual flight information is provided to a trainee-user. Another is for actual in-flight guidance, where visual cues processed from infrared or radar signals are provided to one of the pilot's eyes. Other applications are emerging, for example in architecture, medicine, engineering, and entertainment, as real-time computer-graphics capabilities are developed.
One desired type of virtual reality system employs a head-mounted display (HMD) designed to be worn by a user, and capable of generating visual images which -are perceived as three-dimensional virtual images. Head-mounted displays for use in flight simulation systems have been proposed. For example, U.S. Pat. No. 4,048,653 describes a head-coupled visual display system which includes a head-mounted unit worn by the observer, and a image projection system for projecting left-eye and right-eye images onto left- and right-eye screens in the head-mounted unit, via an optical train. More recently, the present inventors have described a HMD in which visual images are generated by liquid-crystal display (LCD) screens in a head-mounted unit, and viewed by the observer as three-dimensional virtual images.
In general, it is desirable for an observer to experience a wide-angle virtual image, such that even the peripheral vision of the observer is engaged. One aspect in the design of a wide-angle visual system in an HMD is the requirement for a large-diameter lens having a relatively small focal length. Such lenses can be relatively bulky and expensive. Further, spherical aberration effects associated with spherical glass lenses can produce pronounced rectilinear distortion at the outer regions of the wide-angle image. Heretofore, although a variety of head-coupled image systems have been proposed, none of these has proven capable of achieving distortion-free wide-angle image presentation.